MANILA (Reuters) - The Philippines raised the alert level at its Mayon volcano on Monday after a loud explosion in the wake of increased activity made a hazardous eruption likelier, prompting authorities to close all schools and urge residents to stay indoors.

Mayon, the country’s most active volcano, has been spewing ash, lava, and pyroclastic material since Jan. 13, displacing close to 40,000 residents in the central province of Albay.

The Philippine Institute of Volcanology and Seismology (Phivolcs) raised its alert on Mayon to level 4, signifying a hazardous eruption is imminent, from level 3, which warns that such an eruption could be “weeks or even days” away.

“We strongly advise all people, both residents and tourists, to avoid the danger zone, and airlines to avoid flying near the volcano summit,” agency chief Renato Solidum told a news conference.

The danger zone around the 2,462-metre (8,077-foot) volcano has been expanded to a radius of 8 km (5 miles), he added.